Groundwater Contamination in Karst Terranes

Water, Air, and Soil Pollution: Focus (2006) 6: 157–170DOI: 10.1007/s11267-005-9004-3 C© Springer 2006

GROUNDWATER CONTAMINATION IN KARST TERRANES

RONALD T. GREEN1,∗, SCOTT L. PAINTER1, ALEXANDER SUN1

and STEPHEN R. H. WORTHINGTON2

1Geosciences and Engineering Division, Southwest Research Institute®, 6220 Culebra Road, SanAntonio, TX 78228-0510; 2Worthington Groundwater, 55 Mayfair Avenue, Dundas,

Ontario, L9H 3K9, Canada(∗ auhtor for correspondence, e-mail: [email protected], Tel: (210) 522-5305, Fax: (210) 522-5184)

(Received 14 December 2004; accepted 13 November 2005)

1. Introduction

Karst terranes cover 7–10 percent of the earth’s landmass and karst aquifers consti-tute 25 percent of the earth’s groundwater resources. In the United States, 20 percentof the land surface is karst (Figure 1) and 40 percent of the groundwater used fordrinking comes from karst aquifers (Quinlan and Ewers, 1989). The significance ofkarst aquifers as sources for potable water will only be enhanced as the demand forwater increases, water resources are depleted, and safe groundwater supplies areendangered by ill-advised development or by contamination exposure from eitherinadvertent or intentional actions.

Karst aquifers exhibit characteristics sufficiently different from porous mediaaquifers that groundwater resource management technology appropriate for porousmedia aquifers is often inadequate for karst aquifers. Karst aquifer characteris-tics include rapid hydraulic responses, focused channelized flow regimes, and fastgroundwater flow and transport in conduits. In addition, karst features such as sink-holes, losing streams, or direct epikarst systems provide for rapid recharge andminimal, possibly non-existent, natural filtration processes. These characteristicsand features render karst aquifers especially vulnerable to contaminant spills anddegradation when wellhead protection areas are not properly designated. It is im-perative in today’s political environment to recognize that these characteristics andfeatures render karst aquifers particularly vulnerable to degradation resulting fromthe intentional introduction of contaminants onto sensitive recharge areas overlyingthese aquifers.

The particular significance of these risks is illustrated when comparing transportcharacteristics in karst and porous medium aquifers. Groundwater and contaminantscan easily travel more than a kilometer per day compared with velocities in porousmedia that are typically less than a meter per day. The rapid velocities and short

158 RONALD T. GREEN ET AL.

Figure 1. Karst regions of the United States (after Davies and others, 1984, U. S. Geological Survey,National Atlas, Engineering Aspects of Karst)

travel times in karst aquifers are a double-edged sword. On one hand, once in a karstsystem, contaminants can be quickly purged due to the rapid velocities. Conversely,contaminants that enter the fast flow regime of a karst system can quickly arrive at awellfield or be discharged at a spring, oftentimes with little or no advance warning.Consequently, karst water resources are vulnerable to rapid contamination withvery little lead-time. However, once introduced into a karst system, contaminantscan also be rapidly flushed because of the large flow volumes and fast velocities.

2. Historical and Current Characterization and Treatment

of Karst Aquifers

Recent history provides vivid examples of karst groundwater systems that weremisinterpreted as governed by diffuse flow dynamics leading to significant under-estimation of groundwater travel times and inappropriate contaminant transportvelocities. Unfortunately, these practices continue. A historical example and anongoing assessment of contamination of karst aquifers are provided.

2.1. HISTORICAL EXAMPLE: WALKERTON, ONTARIO

In May 2000, seven people died and 2300 people in Walkerton, Ontario became illfrom exposure to bacteria contaminated drinking water (Escherichia coli 157:H7and Campylobacter jejuni)(Worthington et al., 2002). The source of the bacteriawas thought to have been agricultural, or a related activity such as a feed lot. Con-tamination of the municipal water supply system had occurred within hours ordays after heavy rains which suggested the travel time of the contaminants waslimited (Figure 2). The municipal well field was developed in a near-surface lime-stone formation. The original hydrogeological investigation of the site characterized

GROUNDWATER CONTAMINATION IN KARST TERRANES 159

Figure 2. Comparison of illness onset dates and precipitation at Walkerton, Ontario (from Worthingtonet al., 2002)

Figure 3. Trajectories and travel times for the Walkerton, Ontario tracer tests compared with 30-daycapture zone calculated with MODFLOW (from Worthington et al., 2002).

the limestone aquifer as an equivalent porous medium (EPM) (Golder Associates,2000a,b). Numerical model analyses performed with MODFLOW (Harbaugh et al.,2000) predicted a 30-day time-of-travel area that extended about 150 to 290 m fromthree municipal pumping wells (Figure 3). The source of contamination was as-sumed to have originated within the 30-day time-of-travel area due to the limitedviability of pathogens in the subsurface. Following this characterization, the inves-tigation for the source(s) of contamination focused on a limited area around thethree wells.


A subsequent hydrogeological investigation of the site provided an alternativeconceptual model of the Walkerton aquifer (Worthington et al., 2002). The near-surface limestone was characterized by Worthington et al. (2002) as karstic basedon the presence of springs and on discrete flows into the wells from solutionally-enlarged conduits. Both the conduit inflows into the wells and the springs had rapidlyvarying flow and water quality. Aquifer tests of the limestone aquifer at Walkertonindicated hydraulic conductivity values of 1.2 × 10−4 to 1.5 × 10−5 m/s, somewhatgreater than the upper limit of 10−5 m/s typically associated with non-karstic lime-stones. Lastly, tracer tests performed at the municipal wellfield indicated ground-water velocities from 320 to 480 m/day (Figure 3). This last piece of evidence wasinterpreted to provide irrefutable evidence that the limestone at Walkerton is karstic.

The tracer tests also documented rapid flow from surface water in a neighboringstream to one of the municipal wells and from another well to a spring. Thesemeasured velocities were some 80 times greater than velocities predicted using theEPM MODFLOW results. The potential source area for the bacterial contaminationas defined by the tracer tests results and by treating the aquifer as karstic wasconsiderably greater and possibly more irregularly shaped than the potential sourcearea predicted using the EPM simulation. The tracer tests results indicated thatcontamination could have originated from a much greater distance.

Designation of a 150 to 290-m 30-day time-of-travel area as a wellhead protec-tion area around the water supply wells would not have provided protection to thewater supply wells. In fact, designation of a small wellhead protection area aroundthe well field would have had the potentially disastrous effect of providing a falsesense of security that the well field was safe from a reoccurrence of pathogens inthe water supply.

2.2. PRESENT-DAY EXAMPLE: BISCAYNE AQUIFER, SOUTHEAST FLORIDA

Water managers in southeast Florida face increasingly difficult challenges attempt-ing to balance economic development while responding to their responsibility asstewards of the water resources for a continuously growing population. At issue isthe Biscayne Formation, which provides half of the limestone used for constructionin the state of Florida and supplies drinking water to 1.5 million people in southeastFlorida. The Biscayne aquifer is found in the southeast portion of Florida, extendingto Palm Beach to the north and mid-state to the west.

The Biscayne aquifer is mostly exposed at ground surface with little or no sur-ficial confining (and protective) layer or sediments. Consequently, the Biscayneaquifer is mostly under water-table conditions. One of the largest wellfields inFlorida, the Northwest Wellfield, pumps from the Biscayne aquifer to supply150 million gallons of water per day (mgd), which accounts for a major portion ofthe water demand in Miami-Dade County. The wellfield was originally designed tohave an installed capacity of 225 mgd (Miami-Dade Department of EnvironmentalResources Management, 2000).


Mining companies have applied for permits to develop limestone quarries withinone-half mile of the Northwest Wellfield. The quarries would be wet mined to depthsof 25 m, significantly below the water table. After mining activities are concluded,the quarries will remain as lakes exposing groundwater to surface water. The con-cern is that pathogens, such as cryptosporidium or giardia, could be introducedinto the lakes by surface water runoff and subsequently migrate from the lakes towater supply wells, thereby entering the municipal water system. Proponents of thequarry/lake concept contend that the time of pathogen migration from the lakes tothe wells would exceed the viability time of the microorganisms. This assessment ispredicated on the premise that the Biscayne aquifer acts as an EPM with relativelyslow groundwater velocities.

The USGS and the Miami-Dade County Department of Environmental ResourceManagement (DERM) are conducting an investigation to test the premise that theBiscayne Aquifer acts as an EPM. As part of their investigation, a well-to-well tracertest was conducted at the Northwest wellfield in April 2003 to measure groundwatervelocity. The two wells were separated by 100 m. The apparent mean velocity ofadvective flow (366 m/day) greatly exceeded a simulated velocity of 8 m/day inwhich the medium was characterized as an EPM (Cunningham et al., 2003). Therapid velocity of the dye strongly supports characterizing the Biscayne aquifer askarstic. In addition, hydraulic conductivities in excess of 3.5 × 10−2 m/s have beendetermined by Fish and Stewart (2001) for the Biscayne aquifer which indicatesthe limestone is indeed karstic. This assessment of the Biscayne Aquifer as karsticis supported by analyses by Cunningham et al. (2004), Vacher and Mylroie (2002),and Langevin (2001). Based on these analyses, characterization of the Biscayneaquifer as an EPM is not a realistic assessment.

There is currently a moratorium placed on the issuance of additional miningpermits. The moratorium is set to expire in mid-2005, at which time the resultsof extensive hydrogeological testing of the Biscayne aquifer, including the tracertest results, should be available. Although politically charged, the disposition ofthe mining permitting issue will certainly be affected by the technical assessmentof groundwater flow through the Biscayne aquifer.

3. Security of Karst Water Supplies

The current political and sociological climate dictates that the potential for biolog-ical or chemical attack on water supplies must be considered (Burrows and Renner,1999). Karst water supplies are especially vulnerable to terrorist attacks because oftheir propensity for rapid recharge and fast travel times. Toxic substances could beintroduced into a municipal karst aquifer system resulting in rapid degradation ofthe water resource with high risk consequences to the public who rely on the supply.

Malcolm Field (2002) of the US EPA has recommended a program to providewater resource managers a method to evaluate toxic release scenario simulations,


particularly for application to karst systems. Field notes that water resource man-agers need to have a preparedness strategy in place to guard against terrorist attackson vulnerable water systems. Field recommends that tracer tests be conductedthroughout the subject area for the purpose of defining recharge areas, solute-transport properties, and probable arrival concentrations.

Once a toxic release is known or believed to have occurred, tracer analysis canbe used to predict toxic substance arrival times, dilution, and arrival concentrations(Mull et al., 1988; Worthington and Smart, 2003). When combined with a risk-assessment analysis, the combined information allows water managers to developa set of alternatives to protect the public health. As evident in this discussion, thecentral component to the development of meaningful simulations of toxic transportis representative characterization of karst flow regimes using tracer testing.

4. Characterization of Flow and Transport in Karst Aquifers

As illustrated by the two earlier examples, determination of whether an aquifer iskarstic can be challenging (Schindel et al., 1996). The most definitive techniqueto form the determination is tracer testing, although hydrogeological mapping anddischarge balancing can be supportive (Schindel et al., 1996; Quinlan, 1989). Oncean aquifer is designated as karstic, characterization of the aquifer and the subsequentcharacterization of flow and transport through the karst systems can be equally, ifnot more, challenging.

Effective management of groundwater resources relies of the reasonable appli-cation of representative quantitative tools, of which the groundwater flow modelMODFLOW (Harbaugh et al., 2000) is the most commonly used. MODFLOWis recognized to be an effective model when representing relatively slow laminarflow through porous media. Difficulties arise, however, when attempting to ap-ply a porous media flow model, such as MODFLOW, to aquifers, which exhibitrapid flow potentially under turbulent conditions. This latter flow regime is what isfrequently encountered in the conduit network in karst aquifers.

Clearly, a groundwater flow model that accurately replicates groundwater veloc-ities and solute transport through karst conduits is required if contaminant spills,resource development, wellhead protection, and water resource security of karstaquifers are to be properly managed. Following is a brief summary of possibleapproaches used to represent conduit flow in a karst aquifer, concluding with dis-cussion of a newly developed conduit flow model.

4.1. REPRESENTATION OF CONDUITS IN GROUNDWATER MODELS

Flow in karst systems can occur in at least three modes. Matrix flow, or flow inthe (primary) intergranular pore space, is laminar and generally accepted to bewell described by Darcy’s law. Laminar flow through secondary openings (joints,fissures, faults, and bedding plane partings) also occurs in many carbonate aquifers.


Figure 4. Sensitivity of spring discharge to the diffuse-conduit interaction parameter α0 for a numer-ical test case (taken from Painter and Sun, 2004a,b)

The secondary and matrix flows can usually be lumped together and referred to asdiffuse flow. Conduit flow occurs along solution features and may be laminar orturbulent, depending on the conditions. Worthington et al. (2000) gathered data onseveral carbonate aquifers where extensive caves have been found. Their analysesshowed that the matrix provides over 90 percent of the storage of a karst aquifer,however, more than 90 percent of the flow is through conduits. Thus, it is importantto be able to model the flow in conduit networks and the mass exchange mechanismbetween conduits and the porous matrix. The velocity contrast between the leastpermeable and the most permeable parts in a channeled aquifer are often 6 to 10orders of magnitude (White and White, 2003).

Karst aquifers have been modeled in the past using response function ap-proaches, lumped parameter models, and distributed parameter models. The re-sponse function and lumped parameter models do not provide information on po-tentiometric surfaces and are generally limited to the predictions of spring flows.Distributed parameter models can be classified as: embedded discrete-conduitmodels; single-continuum, smeared-conduit models; and dual-conductivity models(Figure 4).

There are two general conceptualizations of a conduit system employed in dis-tributed parameter models (Figure 5). The conduits can be either thought of as pipesor considered as narrow zones of high transmissivity. In the former approach, eachconduit is modeled as an equivalent pipe with a given cross section, transmissivity,and hydraulic radius. These properties may vary along the conduit. Mass exchangebetween the pipe and the diffuse system is proportional to the flow-wetted area andis not controllable independently. In the latter approach, flow is conceptualized asoccurring in a limited number of conduit zones. Each zone may contain multipleconduits. Rather than resolve each of these conduits, the entire zone is modeled asan equivalent highly transmissive region.

4.1.1. Single-Continuum, Smeared-Conduit ModelsSmeared-conduit models do not attempt to model detailed geometry of relativelysmall conduits. Instead, this approach attempts to capture the effects of a conduit


Figure 5. Grids used in a numerical experiment generated with FEFLOW (left) and MODFLOW(right) (taken from Painter and Sun, 2004a,b)

on a much larger computational grid block by assigning effective properties to thatgrid block. In particular, grid blocks intersected by a conduit are assigned a largevalue for hydraulic conductivity, effectively smearing the conduit across the largergrid block. The approach can be implemented in a finite-element or finite-differencecode, but is primarily applied to structured finite-difference codes. The standardMODFLOW model can be used to implement a single-continuum, smeared-conduitmodel, and several examples exist of this approach applied to the modeling of karstaquifers (e.g., Scanlon et al., 2003).

Experience from fractured rock modeling suggests the smeared-feature approachis adequate for steady-state analysis. However, questions remain about the accuracyof smeared-conduit models applied to transient simulations. In particular, the ac-curate modeling of transient flows and contaminant transport requires the conduitcells be assigned relatively small values for storage properties. This assignmentmakes it possible to represent flashy behavior in the conduit system, but storagein the diffuse system is effectively ignored for those cells intersected by a con-duit. It is not clear if interactions between the fast-responding conduits and theslower-responding diffuse system are adequately represented by relatively largecomputational cells that combine the small-volume, high-permeability conduitswith the diffuse flow system.

Results of the numerical experiments show the smeared-conduit approaches aresurprisingly flexible and can mimic the discrete-conduit models for a wide rangeof conditions. However, the single-continuum, smeared-conduit approach becomesinaccurate if the density of conduits is large. In addition, standard MODFLOWdoes not include the capability for modeling turbulent flow, which is considerednecessary for adequate representation of flow dynamics within conduits.


For unconfined conditions, the approach becomes unwieldy and of question-able accuracy even for sparse conduit networks. Specifically, the transmissivityof a conduit depends on the hydraulic head through the saturated thickness, andcalibrating the conduit transmissivity and storage properties may not work acrossthe entire range of hydraulic heads. Moreover, the single-continuum approach doesnot offer good control of the flow between conduits and background, an importantcalibration parameter.

4.1.2. Embedded Discrete-Feature ModelsCompared with the finite-difference method, the finite-element method is bettersuited for representing complex geometrical shapes. Automatic mesh generationand mesh adaptation are offered by most modern finite-element software. In ad-dition to offering better geometry representation, mesh adaptation allows creationof locally refined meshes at regions where the highest accuracy is desired, whilekeeping coarser discretization elsewhere in the model. This capability for con-structing irregular finite-element meshes makes embedded discrete-conduit modelspossible.

Embedded discrete-conduit models are the most literal representations of con-duits within a groundwater flow model. The basic approach is to explicitly rep-resent each conduit within the finite-element mesh, with mesh refinement in thevicinity of each conduit. The conduits themselves may be represented as specialone-dimensional (1D) line elements or as 2D “ribbon” elements. For example,Kiraly (1998) represented high conductivity karst conduit network by 1D elementsembedded in a three-dimensional (3D) mesh representing the diffuse flow system.The commercial finite-element software FEFLOW (Diersch, 2002) has capabilitiesfor implementing the embedded discrete-conduit model. In addition, finite-elementcodes are not directly compatible with legacy groundwater modeling data, whichare in the MODFLOW format. Also, field methods for determining positions anddetailed geometry of conduits are not available

4.1.3. Dual-Conductivity ModelsIn the single-continuum, smeared-conduit and the embedded discrete-conduit mod-els, a single heterogeneous flow system is represented. An alternative is to modeltwo overlapping and interacting flow systems, the dual conductivity model. In thisapproach, the diffuse background system is modeled as a continuum. Coupled tothat continuum, and exchanging fluid with it, is a network of conduits. If the con-duit network is dense, the conduit system can be modeled as a continuum, andthe approach is identical to the dual-continuum model that has long been used infractured rock modeling. For mature karst aquifers, a relatively small number ofconduits may dominate the flow, in which case the conduit system is a sparse net-work and not a continuum. Linear exchanges between the conduit system and thediffuse system are assumed, irrespective of whether the conduit system is modeledas a sparse network or a continuum. In this sense, the general approach is closely


related to the classical dual-continuum model originally developed for fracturedrock (Barenblatt, 1960).

The dual conductivity model has been applied to karst modeling with the conduitsystem represented as a continuum (Teutsch and Sauter, 1998). The CAVE module(Liedl et al., 2003), developed for use with MODFLOW-96, represents a partialimplementation of the dual conductivity model with the conduit system representedas a discrete network. The CAVE module does not include transient capability forthe conduit system and is limited to confined aquifers.

4.2. DUAL-CONDUCTIVITY MODFLOW MODEL

A new dual-conductivity model, MODFLOW-DCM, was recently developed fortesting the dual-conductivity approach (Painter and Sun, 2004; Painter et al., 2004).The new model is a modification of standard MODFLOW. The MODFLOW-DCMmodel provides more modeling flexibility compared with the single-continuum,discrete-conduit approach (i.e., standard MODFLOW) and was able to reproducethe discrete-conduit results in numerical experiments. In addition, MODFLOW-DCM has an additional calibration parameter, the matrix-conduit exchange termwhich allows the model to be calibrated for different matrix-conduit hydraulicrelationships.

MODFLOW-DCM is currently limited to a single-layer aquifer. Specifically,the diffuse background is modeled as a single-layer aquifer, and the conduit systemis assumed to be positioned within this single-layer aquifer. The approach incorpo-rates significant three-dimensional information, despite the single-layer limitation.Specifically, the aquifer top and bottom and the top and bottom of the conduitsystem are input as functions of the x-y position, a capability that is required torealistically incorporate unconfined conditions.

Sensitivity to the exchange coefficient, α0, is explored in Figure 4. Spring hydro-graphs (discharge versus time) are shown for three different values of the couplingcoefficient α0. The recharge event is a square pulse with duration of 2 hours andmagnitude of 0.5 m3/s. For small values of α0, the interaction between the conduitand the diffuse system is relatively weak, and the spring discharge responds rapidlyto the recharge event. As α0 increases, more water is transferred to the diffusesystem, and the spring hydrograph responds slower and has a slower recession.This sensitivity to the exchange coefficient indicates that it is an important param-eter for use in calibration. Numerical tests indicate the hydrograph becomes nearlyindependent of conduit storage if α0 is sufficiently large.

Numerical experiments were performed (Sun and Painter, 2004) to evaluate thepotential of MODFLOW-DCM to replicate flow through a karst aquifer. Simulationresults were compared with results from an embedded-pipe model simulated usingFEFLOW (Figure 5). The configuration for the finite-element grid generated withFEFLOW is compared with the finite-difference grid generated with MODFLOW-DCM in Figure 6. Hydraulic heads along the transect y = 387.5 m are shown for two


Figure 6. Hydraulic head and spring discharge as calculated by MODFLOW-DCM and FEFLOWfor the multiple conduit example of Figure 5. Shown are the (a) steady-state hydraulic head alongthe scan line y = 387.5 m (see Figure 5), (b) hydraulic head along the same scan line after a 2-hourinfiltration event, and (c) spring discharge versus time (taken from Painter and Sun, 2004a,b).

10

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0 200 400 600 800 1000

Dist (m)

)m(

da

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Figure 7. Comparison of the head distributions along a cross section drawn at y = 387.5 m for thenumerical experiment illustrated in Figure 5. The FEFLOW result is shown as a solid line, and theMODFLOW result is shown in the line with squares. The head distributions at steady state (left) andhead distributions at the end of the infiltration event (right) (taken from Painter and Sun, 2004a,b).

times: at steady state and after 2 hours of recharge into the conduit system (Figure 6).Also shown is spring discharge versus time. The MODFLOW-DCM simulation re-produces the FEFLOW simulation much better than the single-continuum standardMODFLOW (Figure 7). This better performance is due, in part, to the additionalcalibration parameter α0, which is not available in a single continuum run, and inpart, to the fact that the dual-conductivity simulation does not artificially reducethe diffuse storage as does the single conductivity simulation.


5. Conclusions

Effective management of porous media aquifers can be addressed using groundwa-ter flow and transport tools based on Darcy’s law, appropriate for relatively slowlaminar flow. Karst aquifers, however, pose special challenges when attemptingto characterize groundwater flow and transport. Unless the rapid flow and trans-port of groundwater through conduits are accurately represented in modeling tools,predicted groundwater velocities and travel times will be greatly under-predicted.

Contamination of a municipal wellfield in Walkerton, Ontario and the potentialimpact of surface mining activities near an existing municipal wellfield in Miami-Dade County, Florida, provide examples illustrating the challenges when attemptingcharacterize groundwater flow and transport through karst media. In addition, thereexist realistic threats that the water resources of karst aquifers could be intentionallydegraded by the introduction of toxic substances into sensitive zones of a karstaquifer.

Effectively addressing resource management of karst systems requires: (i) theaccurate characterization of the karst system, particularly in terms of recognizingthe presence of fast-flow conduits and ideally, the dynamic response of the systemto hydraulic boundary effects and changes in hydraulic conditions (i.e., precip-itation) and (ii) application of a karst modeling tool that accounts for fast flowthrough conduits, large storage capacity of the matrix, and appropriate hydrauliccommunication between the matrix and the conduit system.

A recently developed modeling package, MODFLOW-DCM (Painter and Sun,2004: Painter et al., 2004; Sun and Painter, 2004), provides simulation of realisticflow through karst conduit systems. The new package is built upon the existingMODFLOW code. This allows for ease of use when applying the model to existingMODFLOW models. MODFLOW-DCM allows for either laminar or turbulent flowthrough confined/unconfined systems. The hydraulic communication between ma-trix and conduits can be calibrated for the particular hydrogeological environment.Numerical experiments illustrate the potential of the code compared with existingmodeling approaches.

6. Acknowledgement

The authors thank Gordon Wittmeyer and David Ferrill for their review commentsand to Julie Baker and Robert Renken for their insight on the Biscayne Aquifer.The paper was improved by these contributions.

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Joint IAH-CNC and CGS Groundwater Specialty Conferences, Niagara Falls, Ontario, Canada,October 20–23, 2002, pp. 1,081–1,086.

Documents

Groundwater Contamination in Karst Terranes